1. Field of the Invention
This invention relates to methods and equipment for continuous or semi-continuous casting of metals.
2. Description of the Prior Art
Continuous casting, or semi-continuous casting as it is frequently referred to in the aluminum industry, is a method used for casting metals such as aluminum and steel from molten metal. The advantages of continuous casting include a relatively low expense and a higher yield than ingot casting, as well as the ability to directly form slabs that would otherwise have to be formed from ingots by rolling. Two methods of continuous casting are used in the aluminum industry--direct chill (DC) casting and electromagnetic (EM) casting, the former using a water-cooled mold to contain the molten metal as it solidifies, and the latter using an electromagnetic field for the same purpose and eliminating some of the surface defects that arise in DC casting.
In either of these methods, the cast metal moves continuously downward while molten metal is continuously fed to the top of the apparatus. To achieve efficient cooling of the metal, the incoming molten metal is directed by nozzles or distributors to flow in a horizontal direction toward the shorter sides of the mold where the cooling rate is greater.
A typical caster and distributor for aluminum casting is shown in FIGS. 1a and 1b. FIG. 1a shows the horizontal cross section of the caster in dashed lines 11. The cross section is divided into quadrants by two orthogonal bisecting panes 12, 13, that intersect at the vertical axis 14 of the caster. One of the quadrants 15 is shown in solid lines. At the base of this quadrant is the bottom block 16 which supports the aluminum that has already solidified. Molten aluminum is fed to the caster at its top center through a nozzle 17 and distributor bag 18, and cooling of the molten aluminum occurs through the side walls of the caster, forming a solidification front 19.
FIG. 1b is an enlargement of the nozzle 17 and distributor bag 18. The distributor bag is an open-top receptacle whose sides and base are formed of closely woven ceramic or heat resistant fabric that is virtually impermeable to aluminum at the pressure differentials encountered in the metal pool. Its two short sides, however, which are parallel to the two short sides of the casting mold, contain windows 21, 22 where a fabric of a much more open weave is used. The lower end 23 of the nozzle 17 is open and terminates above the base 24 of the bag, allowing the molten metal to flow from the nozzle to the windows and out of the bag.
A typical caster and nozzle for steel casting is shown in FIG. 2, which presents a view identical to that of FIG. 1a, including the full cross section shown in dashed lines 11 and the two orthogonal bisecting planes 12, 13. Distribution of the molten steel, however, is achieved by a cylindrical nozzle 27 or "shroud" without a bag, since ceramic fabric bags are not functional at the high temperatures required for molten steel. Unlike the nozzle used in aluminum casting, the steel distributor nozzle is closed at the bottom and contains two lateral holes 28, 29 at opposite sides of the cylindrical wall close to the closed bottom. Both the windows 12, 13 in the aluminum distributor bag and the two holes 28, 29 in the steel nozzle are intended to direct the flow of the incoming molten metal toward the two short sides of the casting mold, where the cooling rate is greater. The goal is a uniform temperature gradient and solidification rate along the perimeter of the mold. This minimizes solidification defects such as segregation and coarse-grained regions or "cold shuts," and yields better quality castings.
With either type of distributor, however, the metal solidifies in the mold in three regimes, located in segregated regions in the transverse cross section of the cast metal. These are the chill zone located near the mold wall, the columnar zone defined by dendrites that extend inward from the chill zone, and the equiaxed zone surrounding the central axis of the mold. In the equiaxed zone, the grains are closest to being isometric, i.e., equal in dimension along all three axes, and are usually more fine-grained. Studies have shown that the equiaxed zone can be expanded and the grain size decreased by increasing the rate of agitation of the metal as it solidifies. This is desirable because it facilitates subsequent processing of the cast metal. The time needed to homogenize the metal by annealing, for example, is reduced, and rolling of the metal is easier and produces a more uniform product.
The generally accepted explanation of why agitation is successful in yielding a smaller grain size and a larger equiaxed zone is the "fragmentation" of dendrites growing at the solidification front. Fragmentation, or the detachment of dendrite arms from the dendrites, results from fluctuations in the temperature of liquid adjacent to the dendrites, or in the concentration of the liquid, which lowers the local melting point. Once detached, the fragments are swept to other regions of the melt by the flow, where they serve as nuclei for the formation of grains as solidification proceeds.
In the steel industry, agitation during casting is frequently imposed by electromagnetic stirring, which involves the imposition of electromagnetic stirring forces on the melt by a low frequency power supply and inductor. Electromagnetic stirring is costly, however, due to the significant investment associated with the power supply and inductor plus the cost of electricity. Furthermore, because of technical difficulties in imposing electromagnetic stirring at the level of the mold, the stirring is usually applied below the mold. In fact, attempts have been made to reduce the force of the jets of metal flowing from the nozzle holes 28, 29 (FIG. 2) by electromagnetic braking.
In the aluminum industry, electromagnetic casting causes electromagnetic stirring to occur in the metal pool, but only in the few millimeters at the periphery of the liquid pool, whose width is typically of the order of 1 meter. This therefore has little influence on the structure of the cast metal. The main method to date for improving grain structure in the aluminum industry, in either direct chill or electromagnetic casting, has been the addition of "grain refiners" which are chemical compounds (such as titanium diboride, for example) that form nuclei for the solidifying grains. Grain refiners add to the cost of the process, however, and they raise the impurity level of the finished metal.
There remains a need to improve the quality of the cast metal by increasing the size of the equiaxed zone and decreasing the grain size in the zone, as well as by avoiding casting defects such as cracks. These and other problems encountered in the prior art are addressed by the present invention.